A NAND flash memory uses high voltages which are higher than an external power supply voltage at times of write and erase. These high voltages are generated by using a charge pump circuit functioning as a boost circuit. An output voltage of the charge pump circuit is detected by a detection circuit, and the operation of the charge pump circuit is controlled based on an output signal of the detection circuit.
When the charge pump circuit starts to operate, a ripple component occurs in the output voltage. In the case where such a ripple component is to be suppressed, the boost operation delays.